A survey of the urea transport properties of the nephron segments in the kidneys of rats and rabbits is being carried out in order to formulate an integrated model of urea excretion by the kidney. Initial studies were done in rabbit nephrons until it was discovered that nephron segments could be dissected from Sprague-Dawley rats which were specially-maintained to avoid exposure to pathogenic environmental factors. As was true in the rabbit, cortical thick ascending limbs of rats were found to be significantly permeable to urea (apparent permeability: 1.4 x 10-5 cm/sec). Preliminary studies in cortical collecting ducts from rats reveals a permeability to urea which is about half that of the cortical thick ascending limbs. Studies of the proximal straight tubules from rabbits did not confirm another laboratory's finding of active urea secretion.